A light-emitting device in the ultraviolet region is particularly expected significantly in a realization of fluorescent lamps of mercury-free, photocatalysts providing a clean atmosphere, oncoming generation DVDs realizing a higher density recording and the like. In view of such background, GaN blue light-emitting device has been realized.
In Japanese Patent No. 2778405, such GaN blue light-emitting device composed of a sapphire substrate, a buffer layer formed on the sapphire substrate, an n-type gallium nitride compound semiconductor layer formed on the buffer layer, an n-type cladding layer, an n-type active layer, a p-type cladding layer, and a p-type contact layer is described. The prior art GaN blue light-emitting device emits an ultraviolet radiation of 370 nm emission wavelength.
However, it is difficult to obtain a light-emitting device emitting a light having a shorter wavelength than the ultraviolet region because of a bandgap in a conventional GaN blue light-emitting device.
In this respect, it is considered that β-Ga2O3 and ZnO have a possibility of emitting a light having a shorter wavelength than the ultraviolet region, so that it is studied to use these compounds for light-emitting devices. For instance, it is studied to fabricate a β-Ga2O3 bulk single crystal substrate in accordance with CZ method (Czochralski method) or FZ (Floating Zone Technique) method.
On one hand, Japanese Patent Application Laid-Open No. 2002-68889 discloses to glow a ZnO thin film on a conventional substrate in accordance with PLD (Pulsed Laser Deposition) method.
FIG. 27 shows a substrate 170 formed from a conventional β-Ga2O3 bulk single crystal. As a conventional single crystal growing method for manufacturing a material of such substrate 170, the CZ method and the FZ method are known (see “Rev. Int. Hautes Temper. et Refract.” No. 8, 1971; page 291).
The CZ method is conducted in accordance with the manner as described hereinafter.
First, an Ir crucible charged with Ga2O3 having 4N purity is covered by a silica tube, the Ir crucible is heated by a high-frequency oscillator while flowing a mixed gas obtained by adding 1 vol. % of oxygen gas into argon gas through the silica tube to melt a Ga2O3 powder, whereby a Ga2O3 polycrystalline melt is produced. Then, a β-Ga2O3 seed crystal prepared separately is allowed to be in contact with the molten Ga2O3, and the β-Ga2O3 seed crystal is drawn up at a rate of 1 mm/hour with a crystal rotation number of 15 rpm to fabricate a β-Ga2O3 single crystal. According to the present method, there is such an advantage that the β-Ga2O3 single crystal having a large diameter can be grown.
Moreover, FZ method is a manner for growing crystals while sustaining a raw material, for example, a β-Ga2O3 polycrystalline melt positioned on the upper side by a β-Ga2O3 seed crystal positioned on the under side. According to the present method, there are such advantages that a contamination can be prevented because no container is used, that there is no limitation of an atmosphere to be used due to a container applied, and that a material which reacts easily with a container used may be grown.
Furthermore, PLD method is a manner wherein a laser is irradiated to a composition material of a target thin film, for example, a ZnO target in a pulsed manner in an oxygen atmosphere of a very low pressure, so that components constituting the target are made to be in a plasma or molecular state, and the components in such a state are allowed to sputter on a substrate to grow a ZnO thin film on the substrate. Hence, a thin film can be fabricated easily in a simple device.
In a conventional CZ method, however, it is difficult to control a crystal growth, because of a violent evaporation or a remarkably unstable growth of melt components from a Ga2O3 melt.
Besides, although a single crystal of around 1 cm2 is obtained dependent on a condition in FZ method, a twinning tendency or cracking appears because of a violent evaporation and a precipitous temperature gradient from a molten zone, whereby it is difficult to grow the single crystal in size and to make to be high quality. In addition, when the substrate 170 is fabricated with a β-Ga2O3 single crystal a direction of which is not fixed, it is very difficult to cut the crystal in a direction other than the cleavage surface (100), since a cracking 171 appears.
In a thin-film growing method according to a conventional PLD method, ZnO separates from a target made of a composition material of the objective thin film as clusters, and they are deposited on a substrate as they are, whereby ZnO molecules forms irregularities on the substrate, so that there is a fear of forming a thin film having a poor surface flatness. Besides, since there is a case where the target is deteriorated or transformed by laser irradiation, it becomes a factor for hindering a growth of thin-film single crystal.
Accordingly, an object of the present invention is to provide a β-Ga2O3 single crystal growing method wherein its crystal growth is easily controlled, and even if the resulting single crystal is worked into a substrate or the like which has a large size and high quality, cracking appears hardly.
Another object of the present invention is to provide a method for growing a thin-film single crystal by which a thin-film single crystal with high quality can be formed.
A still further object of the present invention is to provide a Ga2O3 light-emitting device capable of emitting a light having a shorter wavelength than that in the ultraviolet region, and its manufacturing method.